A major source of polyester, such as polyethylene terephthalate (PET), is the recycle of bottles made therefrom. These bottles conventionally are prepared from a polyethylene terephthalate resin with a base overlay cup of high density polyethylene (HDPE) resin. Paper labels, etc. are utilized.
The usual separation plant, such as described in Modern Plastics, April, 1980, page 82-3, crushes the received material, grinds it, and separates fines and paper labels through such as fluidized-bed separation procedures. The remaining materials primarily are polyethylene and polyethylene terephthalate. Further separatory procedures are employed to obtain these two materials as separate streams for individual recycle and reuse.
Recently, a foamed polystyrene wrap-around label has been applied to the PET body. This presents an additional problem, now adding a third major different polymer component to be separated.
Another problem experienced in the reuse of polyesters, whether obtained from PET bottle recycling or other, has been the fact that the thermoplastic polyesters are subject to hydrolysis in the presence of moisture when in the molten state during (re)processing. This hydrolysis results in a marked decrease in molecular weight, which then is reflected by marked lowering of physical properties, particularly toughness. To prevent hydrolysis during (re)processing, it has been necessary to dry the polyester to a moisture content of less than such as about 0.005 weight percent or less, a difficult procedure without deteriorating the polyester.
Needed are improved methods of reuse of the tremendous quantities of PET available in the form of recycle material. This material is either from recycle of the tremendous use of PET bottles, or from scrap in primary polyester bottle forming and other purposes. Just for example, the above referred to Modern Plastics article indicated an estimated 1.5 billion PET soft-drink bottles shipped during 1979, accounting for an approximate total of 150,000 metric tons of resin in that single usage. In the intervening time, this likely has doubled; presenting a large potential area for useful recycle. Additionally, probably equivalent quantities of PET are used elsewhere in many other applications presenting equivalent opportunities for recycle.